O-Phenylphenol react with sulfuric acid, formaldehyde and ferric iron to form a red colour compound.

"Food additive" means any synthetic compound or natural substance put into food to improve its quality, colour, fragrance or taste, or for the sake of preservation or processing.

The title compound has a reversible photochromic property. A possible photochromic mechanism was proposed based on the ESR spectra of colour-changed samples.